(svn r8050) -Codechange: Rename map member extra to m6, since its usage has been widden.

Only doc landscape_grid.html has been updated, landscape.html will be done latter.
-CodeChange: Replace a direct access to m6 with SetTropicZone in InitializeLandscape, as it is the only part of m6 still untouched without the brute-forced m6=0

src/void_map.h

@@ -12,7 +12,7 @@ static inline void MakeVoid(TileIndex t)
 	_m[t].m3 = 0;
 	_m[t].m4 = 0;
 	_m[t].m5 = 0;
-	_m[t].extra = 0;
+	_m[t].m6 = 0;
 }
 
 #endif /* VOID_MAP_H */